On July 21, 2001, at 1331 mountain daylight time, a Pilatus PC-6, airplane, N2TS, sustained substantial damage during the landing roll-out at Great Falls International Airport, Great Falls, Montana. The airplane is registered to N Bar Ranch, LLC, and was being operated as a visual flight rules (VFR) personal/pleasure flight under the provisions of Title 14, CFR Part 91, when the accident occurred. The airline transport pilot, and two passengers aboard the airplane, were not injured. Visual meteorological conditions prevailed and no flight plan was filed for the cross-country flight. The flight originated from a private airstrip near Wolf Creek, Montana, approximately 15 minutes prior to the accident. According to a written report submitted by the operator, the pilot was landing the tail-wheel equipped airplane on runway 03. Shortly after touchdown, during the roll-out, the pilot manually unlocked the airplane's tail wheel. Upon releasing the tail-wheel lock, "...directional control of the airplane was lost." The airplane subsequently ground looped resulting in substantial damage to the right wing. The 1257 METAR observation at Great Falls reported winds from 070 degrees at 6 knots. The accident report submitted by the operator indicated that there were no mechanical malfunctions or failures with the aircraft that may have contributed to the accident.